Botanical Studies Based on Textual Evidences in Eastern Asia and Its Implication on the Climate

1. Introduction

Climate change has long been acknowledged as a major cause of biodiversity change, as well as historical and cultural developments [1,2]. High-quality paleoclimate reconstructions can provide a clear climatic context for comparing and forecasting climate change trends and their impacts, particularly in the last 2000 years, when China experienced a rapidly growing population, expanding and intensifying agriculture, cultural diffusion, and climate fluctuations but lacked a fossil record of climate [3,4]. Some research implied that the climate was 2.5°C cooler than it is now before 1500-1000, while others suggested that it was warming [4,5,6]. One key explanation for these inconsistencies, if not contradicting conclusions, was the scarcity of solid quantitative climate reconstructions, particularly in China’s cultural heartland.

Climate influences plant distribution, assemblages, and morphological features [7,8]. The distribution of modern plants, which are commonly used to quantitatively study paleoenvironmental and paleoclimatic features, such as endemic species climate analysis, coexistence approach (CA), leaf phase analysis, and correlation between plant stomatal parameters and palaeoatmospheric CO₂ concentration changes, can reflect plant climate tolerance [9,10]. One of the plant estimating climate approaches is the Mutual Climatic Range (MCR), which estimates the range of climatic parameters that plants may coexist with by analyzing the species assemblages within the plant community. MCR has been frequently used to reconstruct climate parameters from faunal, spores and pollen, and plant macrofossil assemblages since the Quaternary [6,11]. Notably, the “Coexistence Approach” (CA) use the MCR in conjunction with the nearest living relative (NLR) principle to calculate the maximal climate tolerance of the fossil taxonomic unit NLR based on reconstruction of climatic factors since the Cenozoic [9].

The quantitative reconstruction of the MCR paleoclimate requires the study and identification of all components of the palaeobotanical assemblage. Natural evidence of changes in various types of climatic environments, such as tree rings, stalagmites, ice cores, and lake sediments, is typically recorded at long ages and relatively short time intervals [12], making quantitative studies of specific historical periods in the recent past difficult. Empirical research is required to better comprehend the Holocene flora. Although China’s long history and continuous, well-preserved written records, as well as the accumulating archaeological finds, provide a unique opportunity to verify detailed plant-climate information across a wide geographic range [2], there are yet no appropriate quantitative analyses using these historical data sources.

Henan Province, located in China’s central plain, was ruled by the Wei-Jin-North and South dynasties about 1500 years ago, during which time there was social turmoil, numerous regime changes, and the establishment of temple gardens, which substantially expanded plant species diversity, particularly in the western city of Luoyang [13]. Luoyang is located in a transition zone between semi-humid and semi-arid climates, and is heavily influenced by the North Atlantic movement. According to studies, a chaotic period of low climate stability occurred between 280 and 1230 years ago, during which a rich biodiversity flourished despite extreme climate swings [14,15]. However, quantitative paleoclimate data flora for most of the Cenozoic in China are mostly unavailable, and climate change remains unquantifiable.

A literature survey and the mutual climate range method (MCR) were used in this study to create a quantitative reconstruction of the flora of the Luoyang region. Historical documents from 1475 years ago were examined, the recorded woody plants of the Luoyang region were settled, the respective survival environments of these woody plants were drawn, the climate of the ancient Luoyang region with simultaneous survival of these woody plants was counted using the mutual climate range approach, and the coexistence analysis were used to discuss the climate evolution of China’s western Henan region 1475 years ago.

2. Materials and Methods

2.1. Study Site

Yang Xuanzhi, a Chinese official and literati during the Northern Wei Dynasty. He served as an official in Luoyang from 528 to 530 A.D. More than 10 years later, from 543 to 547 A.D., after rushing back to Luoyang and traveling through the city, Yang Xuanzhi completed a comprehensive notebook documenting the city of Luoyang at that time, named as the “Buddhist Temples in Luoyang”. Modern archaeological research had concluded that the site of the ancient city of Luoyang of the Han and Wei Dynasty, located at 15 km east of Luoyang City, Henan Province, is the Luoyang City of the Han and Wei dynasties as recorded in the Luoyang Galanji [16].

The site studied in this paper is the ancient city of Luoyang of the Han and Wei Dynasty, located in eastern China, in the middle and lower reaches of the Yellow River, 15 km east of the modern city of Luoyang (E 112.45942°, N 34.62426°), which was the capital of seven Chinese dynasties from the 1st to the 6th century AD. As a capital or important city, the ancient city of Luoyang of the Han and Wei Dynasty was used for more than 1,600 years from the establishment of the city in Zhou Dynasty till the beginning of the Tang Dynasty. Archaeological research has revealed that the ancient city of Luoyang of the Han and Wei Dynasty reached its maximum during the Northern Wei Dynasty (386-534 AD), and was divided into a triple city circle of palace, inner city, and outer city, covering an area of 100 square kilometers, making it the largest capital city in ancient China, nine times larger than Constantinople, the capital of the Byzantine Empire, and 8.2 times larger than the city of Baghdad, built in 800 AD [17,18,19] (Figure 1).

2.2. Research Objects and the Time

In the book of “Buddhist Temples in Luoyang”, 23 species of woody plants were recorded in Luoyang City during the Northern Wei Dynasty, 1475 years ago. These woody plants and the climate of Luoyang before 1475 years ago are the researching objects in this study.

The flora ingredients of the Luoyang area in this period were clearly recorded in the book of Buddhist Temples in Luoyang. The study revealed that a total of 23 species of woody plants were described in the book, among which five species of woody plants were not native to Luoyang, namely, apricot, wine tree, noodle wood, oxalis, and dog bone. In addition, there are 18 species of herbs as well as vines and one species of fungus described in the book.

Other records related to woody plants in the Luoyang area have also been studied. Due to the long period of time, there is very little information available for reference. Even for the wooden coffins found in the archaeological excavations in the Luoyang area, the source of the selected materials could not be identified as coming from the local area of Luoyang. It is believed that the choice of material for the coffins was more from the density properties of the wood and the ritual properties given by man [19,20,21,22].

2.3. Taxonomic Study

Taxonomic research work was carried out to determine the taxonomic position of these 23 woody plants using the method of phytological examinations [23,24,25,26] (Figure 2, Table 1 and Table 2)

2.4. Climatic Analysis

In order to reconstruct the local climatic environment using the distribution data of the flora, Mosbrugger and Utescher [27] used the method of the co-existence approach for their research work and gradually built up the Palaeoflora database in the course of continuous practice. This database contains more than 800 fossil plant taxa, extant nearest relative taxa of fossil plants and their climatic elements distributed in Europe. This information is appropriate for analyzing the climate of continental Europe, but less appropriate for analyzing the climate of Asia. Fortunately, the Chinese have constructed a distribution and climate database Atlas of the Distribution of Woody Plants in China based on native plants. With the exclusion of anthropogenic perturbations, i.e., exotic plants, the climate data of the pre-1475 Luoyang area can be derived by bringing the native plant data into this database, deriving the climate data based on different woody plants, and calculating the coexistence intervals among them[10,27,28,29,30] (Figure 3).

3. Results

3.1. Taxonomic Work on Plants

According to the records, the study found that 23 types of woody plants existed in the Han and Wei Luoyang city area 1475 years ago. The study identified 3 species of gymnosperms, namely Pinaceae and Cupressaceae, one species of Tamaricaceae monocotyledon, and 19 species of dicotyledons. However, three plant groups could only be identified to the level of genus. As is shown in Table 1, Figure 2, and Table 2.

3.2. Climatic Analysis

Climatic information regarding the ancient city of Luoyang during the Han and Wei dynasties was compiled by combining records from the Atlas of Woody Plant in China, Distribution and Climate. The collated plant taxa of the area was used as a basis for this. To make a comparison, some typical cities in East Asia were selected and compared with the ancient city of Luoyang during the Han and Wei dynasties. This is depicted in Figure 3, Figure 4, and Figure 5.

3.3. Comparison of Temperature Parameters

Based on quantitative calculations of meteorological data, the meteorological values of the ancient Luoyang of Han and Wei dynasties are not the same as those of modern Luoyang. In the section on temperature, although the mean temperature of the coldest month (MTCM) in the ancient city of Luoyang in the Han and Wei dynasties (2.1°C) was slightly higher than that in the modern city of Luoyang (0. 8°C), the mean annual temperature (MAT) (14.3°C) and the mean temperature of the warmest month (MTWM) (25.3°C) in the ancient city of Luoyang of the Han and Wei dynasties were lower than those in the modern city of Luoyang (14.3°C and 27°C respectively, Figure 4 and Figure 5).

Comparative analysis showed that there were two locations with the same value of 14.3°C as the MAT of the ancient city of Luoyang in Han and Wei dynasties. One was in Hanzhong, Shaanxi Province, and the other was in Fangxian, Anhui Province. Only, Hanzhong is more southward in latitude (1.58°) and more westward in longitude (5.45°) than the ancient city of Luoyang in Han and Wei dynasties. Fangxian was farther south in latitude (2.56°) and west in longitude (1.72°) than the ancient city in Luoyang of Han and Wei dynasties. Meanwhile, although the mean annual temperature of these two cities is the same as that of the ancient city of Luoyang in Han and Wei dynasties, their mean coldest and mean hottest monthly temperatures were different from those of the ancient city of Luoyang of Han and Wei dynasties.

The study results yielded a coldest Mean Temperature of Coldest Month (MTCM) of 2.1°C for the ancient city of Luoyang of the Han and Wei dynasties, which was higher than that of the modern Luoyang region (0.8°C). There are three modern cities that are very close to the coldest monthly mean in the region. The MTCM of Huoshan City in Anhui province was the same as that of the ancient city of Luoyang of the Han and Wei dynasties, however, it was about 3.7194° east and 3.3180° south to that of the of Luoyang of the Han and Wei dynasties. The other two cities are Xinyang in Henan Province and Nanjing in Jiangsu Province, with a 2°C MTCM for the both. Meanwhile, Xinyang City was 1.4626° east and 2.5801 south to that of the ancient city of Luoyang of the Han and Wei dynasties. Nanjing city was 6.1675° east and 2.6689° south to that of the ancient city of Luoyang of the Han and Wei dynasties. (Map: Huoshan, Xinyang, Nanjing) (Figure 4, 5)

The Mean Temperature of Warmest Month (MTWM) of the ancient city of Luoyang of the Han and Wei dynasties was 25.3°C, which was lower than that of the modern Luoyang region (1.7°C). Comparison disclosed that there was only 0.1 degree difference on the MTWM between that of Fangxian in Hubei Province and the ancient city of Luoyang of the Han and Wei dynasties. However, Fangxian was about 2.56° south and 1.72° west to the ancient city of Luoyang of the Han and Wei dynasties. At the same time, the MTWM of Chengdu in Sichuan Province was about 0.3degree higher than that of the ancient city of Luoyang of the Han and Wei dynasties, however, Chengdu was about 3.96° south and 8.39° west to the ancient city of Luoyang of the Han and Wei dynasties. (Fangxian, Chengdu)

Quantitative calculations showed that the humid of the ancient city of Luoyang of the Han and Wei dynasties was slight larger than that of modern Luoyang City, just because the three value of the Annual Precipitation (AP, 1051 mm), the Precipitation of the Coldest Quarter (PCQ, 84 mm) and the Precipitation of the Warmest Quarter (PWQ, 472 mm) are all larger than those of modern Luoyang City (599.5mm, 36.7 mm and 300.5 mm respectively). (Figure 4, 5)

The annual precipitation (AP) of the ancient city of Luoyang in the Han and Wei dynasties was almost twice than that of modern Luoyang City, while the AP of Nanjing City (1070.9 mm) was quite close to that of the ancient city of Luoyang in the Han and Wei dynasties. However, Nanjing was about 6.1675° east and 2.6689° south to the ancient city of Luoyang of the Han and Wei dynasties. At the same time, the AP of Nanchong city in Sichuan province was about (954.9-1051) less to the ancient city of Luoyang in the Han and Wei dynasties, however, Nanchong was (30.84-34.62426) south and (106.12-112.45942) west to the ancient city of Luoyang in the Han and Wei dynasties (Map: Nanjing and Nanchong) (Figure 4, 5)

The precipitation of coldest quarter (PCQ) of the ancient city of Luoyang in the Han and Wei dynasties (84 mm) was larger than that of modern Luoyang City (36.7 mm). The nearest value was that in Haikou, Hainan Province (83.3 mm), however, Haikou was about 2.2849° west and 14.7169° south to the ancient city of Luoyang in the Han and Wei dynasties. (Map: Haikou) (Figure 4, 5) The precipitation of warmest quarter (PWQ) of the ancient City of Luoyang in the Han and Wei dynasties (472mm) was larger than that of modern Luoyang City (300.5 mm). Comparison showed that the value of Laoting in Hebei Province was just the same as that of the ancient city of Luoyang of the Han and Wei dynasties. However, Laoting was about 6.284° east and 4.6983° north to the ancient city of Luoyang of the Han and Wei dynasties. At the same time, the PWQ of Xuzhou was slightly less (3.2mm) to that of the ancient city of Luoyang in the Han and Wei dynasties, however, Xuzhou is 0.47° south and 4.65° east to the ancient city of Luoyang in the Han and Wei dynasties. Meanwhile, the PWQ of Chengdu was slightly larger (6.2mm) to that of the ancient City in Luoyang of the Han and Wei dynasties, however, Chengdu is 3.96° south and 8.39° west to the ancient city in Luoyang of the Han and Wei dynasties. (Map: Laoting, Xuzhou, Chengdu) (Figure 4, 5) In general, result showed that the temperature of Luoyang area 1.475 millennia ago was slightly cooler than that in the mean value and the warmest months, while the temperatures in the cold season are slightly warmer than modern values in the Luoyang area. At the same time, result also showed that the humid in that area 1.475 millennia ago was significantly larger than modern values, hint that the area was wetter 1.475 millennia ago than it is in modern times.

In general, result showed that the temperature of Luoyang area 1.475 millennia ago was slightly cooler than that in the mean value and the warmest months, while the temperatures in the cold season are slightly warmer than modern values in the Luoyang area. At the same time, result also showed that the humid in that area 1.475 millennia ago was significantly larger than modern values, hint that the area was wetter 1.475 millennia ago than it is in modern times.

4. Discussion

4.1. Climate in East Asia in the 6th Century AD

Earlier research concluded that around the sixth century A.D., China experienced 19 exceptionally cold winters. This figure indicated that the climate in China in the sixth century A.D. was colder than it is today [31]. Studies of Greenlandic plant remained in Europe and tree chronology in California supported the idea that the temperature in Europe and North America, like that in China, cooled in the sixth century A.D. [32]. It has been proposed that the Eskimos’ southward attack on the ancient Scandinavians in Greenland during the same period as China’s North and South dynasties, and the resulting destruction of native settlements on that island, were influenced in part by the cooling of the climate at that time [33,34]. It has been proposed that the Chinese climate had gone through three cold and dry eras since 2000a. The second cold period, the cold period of the Wei-Jin-North and South dynasties, occurred between the early A.D. 600a A.D., when research estimated that the mean annual temperature was 2-4°C lower than that of modern times [35]. Another study indicated that after the warm climate of the Qin and Han dynasties, China entered a 400-year cold period known as the Qin-Han-North-South dynasty phase, which was 0.5°C colder than our current winter temperatures. The coldest period during this time was the midst of the Northern and Southern dynasties, when the temperature was around 1.3°C lower than it is now [36]. The analysis in this paper concluded that before 1475, the mean annual temperature and the mean temperature of the Warmest Month in the ancient Luoyang region were lower than those in the present Luoyang region, indicating that the region was in a cold phase. The study indicated that climate change in China reached a low point about 440-530 A.D. during the Wei-Jin and North-South dynasties, with the coldest period (481-510 A.D.) when the mean temperature in the winter half of the year was around 1.2°C lower than in modern times. The period between 460 and 520 was relatively wet in terms of dry and wet variations. Only the 221-580 AD monsoon region in east-central China was the driest since the Qin and Han dynasties on a scale of the previous 2000 years [37]. The study in this paper concluded that, at the time of the authors’ writing of the Luoyang book, the flora responded to a climate in which both the mean temperature of the hottest month and the mean annual temperature that were lower than the corresponding modern values, despite the fact that the mean temperature of the coldest month was about 1.2°C higher than the modern one, indicating that the temperature of the Luoyang region was relatively low at the time, and this result was consistent with earlier studies [37].

4.2. Circumstantial Evidence of the Climate in East Asia in the 6th Century AD

Seven eras of wild elephant activity were discovered in China’s Jianghuai Valley from the second half of the fifth century to the first half of the sixth century. The size of the group of the wild elephant sightings ranged from solitary roaming to herd appearances. According to the History of the South Dynasty, hundreds of wild elephants emerged in Dangtu, Anhui Province, in 552 AD, trampling buildings and causing havoc. Some research have revealed that wild elephants, who presently occupy China’s tropical jungles, could reach the Huaihe River basin (N 36°-31°) using Asian elephant trails (Elephas maximus Linnaeus) without the effect of human activities [38]. It has also been proposed that the presence of wild elephants in the Jianghuai region shows that local temperatures were not lower than they are now [39]. However, numerous research revealed that there was no required link between the arrival of wild elephants and warm and cold temperatures, and that the presence of wild elephants did not imply that the local temperature is higher [40]. The study in this paper contends that the presence of wild elephants in Dangtu (E118.48°, N31.55°) in the Jianghuai region should at the very least indicate that precipitation in the region is relatively high, as a humid environment is more likely to provide plant food for wild elephant survival. The linear distance between Dangtu in Anhui and the Han Dynasty city of Luoyang is less than 400 km both in the plain area, hence the conclusion that the annual precipitation in the Luoyang area projected in this research significantly greater than that in modern times is likewise ruled out. This finding is further corroborated by research on Longguan Lake sediments [41] and data from Jianghuai Plain stratigraphic sections [42].

4.3. Climate Discussions in East Asia Over the Past 1.475 Millennia

The East Asian monsoon is currently thought to be the primary driver of climatic change in East Asia [43,44], with research indicated that it dates back to the late Oligocene/early Miocene [45]. According to the study, warm and humid air currents from the Pacific and Indian Oceans transported substantial volumes of precipitation to the interior of East Asia during the summer, resulting in hot and wet summers in most places. Dry and cold air currents from Siberia lead East Asia’s winter climate to be dry and chilly [46,47]. Before 1475, the ancient Luoyang region, located in the interior of central East Asia, had high temperatures and rainy summers and low temperatures and little rainfall in winter, but the numerical performance of the mean annual precipitation, the hottest seasonal precipitation, and the coldest seasonal precipitation was higher than that of the modern Luoyang region, indicating that the water content in the airflow was much higher. Despite East Asia’s largely monsoonal climate, the low was substantially higher than in the current age before 1475. This condition could be caused by one of two factors. One possibility is that before 1475, the total annual mean temperature in East Asia was slightly higher than in modern times, leading the water content in atmospheric clouds to be higher than in modern times. This hypothesis is incongruous with the fact that the annual mean temperature in the Luoyang region before 1475 calculated in this study was lower than the contemporary value. It is also possible that the water content of the atmosphere was higher in East Asia before to 1475, or that clouds were more effective at retaining water. The ground may have been better covered by green vegetation, such as forests, and the water vapor content dissipated into the atmosphere via plant leaves was higher, causing precipitation in the region to be higher [48,49]. Only when human society evolved and the forest was converted to cultivation, many woody plants that had thrived for millions of years were cut down for human construction or burial. Deforestation of woody plants that lasted thousands of years destroyed surface plants, weakened the water retention effect of forests, and gradually reduced the amount of water vapor provided to the atmosphere, causing precipitation in the area to gradually decrease despite an increase in the mean annual temperature, and desertification of some northern areas. This is also validated by the Tongwancheng site investigation [45].

5. Conclusions

The mean temperature of the coldest month in Luoyang 1.475 millennia ago was around 1.3°C higher than that in modern times. The mean temperature of the warmest month and the mean annual temperatures in Luoyang 1.475 millennia ago were lower than the corresponding modern values 1.475 millennia ago. The ancient Luoyang region had high temperatures and rainfall in the summer and low temperatures in winter, but the mean annual precipitation, the hottest seasonal precipitation, and the coldest seasonal precipitation were all higher than those in the modern Luoyang region.A high-resolution example on rebuilding the ecological environment in East Asia in 1.475 millennia ago was carried out.